The present invention generally relates to optical emission spectrochemical analysis, and particularly to an assembly for creating the high temperature plasma required for spectrochemical analysis of a sample.
In optical emission spectrochemical analysis, a high temperature plasma is used to dissociate the molecular bonds of the sample. The plasma is an ionized inert gas, such as argon. The atomization of the sample by contact with the plasma will cause an excitation of the dissociated atoms to the point where an optically detectable emission is generated. The wavelength and intensity of this emission is indicative of the type and concentration of the atomic elements contained in the sample.
There are at least two known techniques for creating the necessary high temperature plasma. The first technique is referred to as an inductively coupled plasma (ICP), because an induction coil powered from a high frequency alternating current (A.C.) source is used to create the plasma. The other technique is referred to as a direct current plasma (DCP), because the plasma is created by an arc across electrically opposing electrodes which are powered from a direct current (D.C.) source.
One of the distinctions between ICP spectrometers and prior DCP spectrometers relates to the way that the sample interacts with the plasma. In ICP spectrometers, the sample is made to pass directly through the middle of the plasma. Whereas, in prior DCP spectrometers, the sample contacts the plasma, but does not penetrate the plasma. As a result of the sample penetration in ICP spectrometers, sample decomposition is more efficient and a higher analytical sensitivity can be achieved.
Additionally, the lack of sample penetration in prior DCP spectrometers also leads to significant corrosion problems in a relatively short period of time. This is because when the sample stream encounters the plasma, the generally corrosive aerosol vapors from the sample steam spray outwardly from the plasma, and the proximity of the anode to the plasma makes it particularly vulnerable to corrosion. This corrosion significantly increases the maintenance costs associated with the electrode assembly of these prior DCP spectrometers. A discussion of the plasma jets for prior DCP spectrometers may be found in the Hildebrand U.S. Pat. No. 4,147,957, and the Elliott et al U.S. Pat. No. 4,009,413.
Accordingly, it is a principal object of the present invention to obtain the analytical advantages of an ICP spectrometer in a DCP spectrometer using a emission source design which also overcomes the corrosion problems associated with prior DCP spectrometers.
It is a more specific object of the present invention to provide a DCP spectrometer emission source which traps the aerosol sample stream and forces it through the plasma.
It is another object of the present invention to provide a DCP spectrometer emission source which creates a plasma having a generally upwardly converging conical shape when viewed from its lower extremity, so that the aerosol sample will be captured in this cone and drawn up through the plasma.
It is an additional object of the present invention to provide a plasma penetrating emission source which can be retro-fitted into existing DCP spectrometers.
It is a further object of the present invention to provide a DCP spectrometer emission source which provides an ionic emission analytical zone that is well isolated from the atomic analytical zone, with both zones in turn well isolated from the highly intense plasma current trains.
It is yet another object of the present invention to provide a DCP spectrometer emission source which provides for analytical zones of increase size for facilitating enhanced optical analysis by the spectrometer.
It is yet a further object of the present invention to provide a DCP spectrometer emission source which efficiently utilizes the plasma energy in desolvating and exciting the sample stream presented to it.
It is yet an additional object of the present invention to provide a DCP spectrometer emission source which can tolerate high solid sample solutions in analyzing for trace elements.
It is still another object of the present invention to provide a DCP spectrometer emission source which creates a highly stable plasma which permits increased analytical sensitivity.